Process for desensitizing metallic powders



United States Patent Ofiice 3,420,695 PROCESS FOR DESENSITIZING METALLICPOWDERS George B. Rice, Trout Creek, Utah, and Robert L. Dow,

China Lake, Calif., assignors to the United States of America asrepresented by the Secretary of the Navy No Drawing. Filed Nov. 9, 1964,Ser. No. 410,031 US. Cl. 117-100 6 Claims Int. Cl. C23c 3/00 ABSTRACT OFTHE DISCLOSURE A method for making particulate metal safe to handle inlarge quantities by coating the metallic particles with an organicelastomeric material. The desensitized metal powders thus formed arerelatively insensitive to electrostatic discharge and practicallynon-toxic to handlers.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

The present invention relates to a process for desensitizing metallicpowders and to the product thereof.

Many powdered metals suitable for use as additives for solid propellantgrains, explosives, pyrotechnics and the like, are sensitive toelectrostatic discharge, are pyrophoric or exhibit toxic propertieswhich make them hazardous to handle safely. These metallic powders arelimited in their application to use in small quantities and are usuallyhandled with expensive, bulky equipment necessary to protect theindividuals involved in working with them. Handling large quantities ofnon-desensitized metal powders are extremely dangerous because ofpossible fire and explosion. Other methods for desensitization ofmetallic powders are known, such as oxidizing a coating on theparticles, and displacement coating with less active metals. Both ofthese methods produce metal powders with lower reaction rate and energyoutput. The present invention provides a simple process fordesensitizing metallic powders which produces a product having improvedphysical and chemical properties.

It is an object of the present invention to provide a process fordesensitizing metallic particles in an economical and efficient manner.

Another object is to produce metallic particles which are safe to handlein large quantities.

A further object is to provide a process for making metallic powdersrelatively insensitive to electrostatic discharge.

Still another object is to provide metallic particles which arerelatively non-toxic to handlers.

Yet another object is to provide coated metallic particles which havemore energy available for oxidizer-fuel reactions.

A still further object is to provide coated powders having a relativelylow air oxidation rate.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same become better understood byreference to the following detailed description.

In accordance with the present invention a basic process is provided forcoating metallic powders with an organic elastomeric material therebyforming an improved desensitized metal powder. The process generallycomprises mixing moisture-free metallic powder with an acetone or othersuitable solution of a polymer, such as a copolymer of vinylidenefluoride and perfluoropropylene (known to the trade as Viton A) to forma slurry, washing the slurry 3,420,695 Patented Jan. 7, 1969 with anonsolvent, such as hexane, to precipitate the Viton on to the metalpowder and repeating the hexane wash once or twice to harden the polymeron the powder particles. The invention will be described moreparticularly with reference to coating zirconium, uranium, beryllium andmagnesium; however, the process is applicable to other metal powderssuch as hafnium and thorium and mixtures thereof. The following examplesare presented to illustrate the invention and should not be consideredas limiting thereof.

EXAMPLE I In order to desensitize zirconium powder the followingmaterials should be on hand: Moisture-free zirconium, a quantity of18-22% Viton A in solution, and a quantity of hexane. Zirconium powderis procured commercially as a water-wet powder because it is highlyflammable in the dry state. It is important to remove the moisture sothat the polymeric material, Viton A herein used, will adhere to thezirconium particles. The moisture was removed by treating about 35pounds of water-wet zirconium with acetone. This was accomplished inthree operations. Three gallons of technical grade acetone was mixedwith the zirconium powder, and after a few minutes the powder settledand the acetone was decanted or siphoned olf; then, another threegallons of the same grade acetone was mixed into the powder residue, andagain decanted off; finally, a two gallon quantity of reagent gradeacetone was stirred into the zirconium powder residue which was decantedoif leaving moisture-free zirconium powder ready for the desensitizationprocess. A quantity of Viton A solution (previously prepared bydissolving suflicient Viton A in acetone to form an 1822% Viton Acontent) was added to the moisture-free Zirconium powder and stirredvigorously for several minutes until a slurry or suspension formed. Thisslurry was washed with hexane with continuous agitation for from 5 to 10minutes, the volume of hexane being from one to four times the volume ofslurry. After the slurry settled the supernatant hexane was decanted orsiphoned off. This step precipitates the Viton on to the zirconiumpowder particles. The Viton-coated powder is washed with hexane in aboutthe same amount as before to harden the Viton on to the metallic powder.After the residue settles, the hexane is decanted or siphoned ofi" andthe residue is air dried. The product produced is a fine, free-flowingViton-coated zirconium powder which may be safely handled in largeamounts without danger of fire or explosions.

The slurry may be washed with normal hexane or other low boiling liquidhydrocarbon two or three times as necessary.

The removal of the moisture from any water-wet metal powder is necessaryfor good adhesion of the polymeric coating. Other drying agents includeany material such as ketones which will pickup water.

The table following shows the sensitivity test results of three batchesof the Viton-coated zirconium powder prepared by this process ascontrasted to the uncoated powder.

EXAMPLE II Uranium is highly pyrophoric and electrostatic sensitive wasdesensitized by the process using Viton A described in Example I.Uranium powder is shipped commercially under acetone for safetypurposes. A weighed amount of uranium was mixed with an 18-22% solutionof Viton for several minutes until a slurry formed which wassubsequently washed with hexane to shock-gel the polymeric material,Viton, to the uranium particles. The product produced was a Viton-coateduranium powder which was free-flowing, and safe to handle in largeamounts in the mixing of propellant materials.

EXAMPLE III Magnesium powder which is highly pyrophoric was alsodesensitized by the process described in Example I. It is shipped underheptane, hexane or kerosene, the excess of which is decanted off andWeighed amount mixed with an acetone solution of Viton A to form aslurry. The slurry was washed with hexane to precipitate or shock-gelthe Viton onto the magnesium particles. Following a second hexanewashing to harden the Viton coating the material was air dried. Theresultant powder was fine, free flowing, and safe to handle.

EXAMPLE IV Beryllium was also desensitized by the process hereindescribed to overcome the dusting danger (inhalation) of this very toxicmaterial. Beryllium is procured as a dry powder (ranging from 12 to15 1. particle size). A weighed amount was mixed with an 18-22% Vitonsolution to form a slurry which was washed with hexane according to theprocedure hereinbefore set out. The Viton-coated beryllium particleswere practically dustfree which greatly reduced the toxicity hazardencountered in handling the uncoated powder.

The powders produced herein consist essentially of from 4 to 6 percentby weight Viton and the remainder metal.

Nylon was used in place of Viton, but did not prove to be as effectiveas desensitizer as the Viton. The nylon was dissolved in a reagent grademethanol, mixed into the selected metal powders to form a slurry, andshocked out with hexane. From /2 to 4 percent by weight nylon wasdeposited onto the metal particles.

In preparing the 18 to 22 percent solution of the copolymer ofvinylidene fluoride and perfluoropropylene a slab of Viton A was placedin a quantity of reagent grade acetone and left for about 16 hours. Thesolution was drained E and tested for Viton content because an 18 to 22%Viton solution was found to be successful.

Other polymeric materials which may be used in addition to the copolymerof vinylidene fluoride and perfluoropropylene (Viton A), include asimilar copolymer of hexafluoropropylene and vinylidene fluoride,generally known as Fluorel, vinylidene fluoride andchlorotrifiuoroethylene, sold under the trade name Kel-F elastomer, andnylon which is a long chain polymeric amide.

What is claimed is:

1. A method for desensitizing particulate metal which comprises:

(a) dissolving a copolymer of vinylidene fluoride and perfluoropropylenein acetone to form a solution having a solids content of from 18 to 22percent by weight of the copolymer;

(b) incorporating particulate metal into said solution to form a slurry;

(c) precipitating the copolymer from said solution onto said metal byadding a quantity of hexane to the slurry thereby forming a coatedmetal;

(d) separating said hexane from the coated metal; and

(e) drying the coated metal.

2. The method of claim 1 wherein the particulate metal is zirconium.

3. The method of claim 1 wherein the particulate metal is uranium.

4. The method of claim 1 wherein the particulate metal is beryllium.

5. The method of claim 1 wherein the particulate metal is magnesium.

6. The method of claim 1 wherein the coated metal is treated with asecond quantity of hexane thereby hardening the coating on said metal.

References Cited UNITED STATES PATENTS Re. 25,277 10/ 1962 Toulmin.

1,915,788 6/1933 Hardy 117100 X 2,770,605 11/1956 Honn 117-16l X2,793,200 5/1957 West 260-92.1 X 3,035,948 5/1962 Fox 14919 3,110,63811/1963 Murphy et a1. 117-100 X 3,133,841 5/1964 Kuehl 149-5 3,202,5578/1965 Kaufman 26092.1 X 3,300,329 1/1967 Orsino et al l17161 X WILLIAMD. MARTIN, Primary Examiner.

U.S. Cl. X.R.

